(a) Field of the Invention
This invention relates to a process and apparatus for producing an extruded thermoplastic polymer material in modified form.
In one particular aspect the invention relates to a process and apparatus for the production of extruded insulating coatings in cellular or cross-linked form for wire and cable.
(b) Description of Prior Art
In the production of wire and cable having an insulating coating, a thermoplastic material is extruded through a screw extruder and around a wire or cable passing through a die in the head of the screw extrusion apparatus. In accordance with the properties required in the coated wire or cable, the thermoplastic material is suitably modified.
For example, if a cellular effect is desired in the insulating coating, then a gaseous material may be incorporated in the thermoplastic material in the screw extruder barrel. The gaseous material may be formed in the thermoplastic material by the introduction of a solid blowing agent which is thermally decomposed to produce bubbles of gas, which provide the cellular effect; alternatively a gaseous material may be introduced directly into the thermoplastic material. In the latter case the introduction of an inert particulate material, for example, clay or metals is also desirable; to provide nucleation sites for the formation of the gas bubbles; if no nucleation sites are provided for the gaseous material there is a tendency for the gaseous material to form a solid solution with the thermoplastic material and no cellular effect is produced or at best the extruded material contains only random cells and lacks uniformity.
These methods have certain disadvantages in that the non-gaseous decomposition products produced in the thermal decomposition of the blowing agent and the properties of the inert particulate material can adversely effect the electrical properties of the coated cable.
For example, a commonly used blowing agent for the production of cellular thermoplastic material is azodicarbonamide, which, in the commercial form, or in the presence of commonly used additives, when thermally decomposed produces water, which if retained in the thermoplastic material will affect the electrical properties of the wire or cable coated with the material.
In most domestic articles the presence of water in the extruded cellular product would be of no significance, however, in the communication cable industry the presence of water in the extruded cellular insulating coatings of the cable conductors represents a serious problem.
The most commonly employed thermally decomposable blowing agent for the production of cellular insulated coatings in the communication cable industry is azodicarbonamide; the azodicarbonamide used by the communication cable industry includes a hydrated silica additive as a plating out agent; this hydrated silica produces water on heating which reacts with isocyanic acid produced in the thermal decomposition of the azodicarbonamide to prevent formation of cyanuric acid and cyamelide which would otherwise build up on the screw and die surface in the form of a white powder or pasty substance and interfere with the satisfactory operation of the extrusion, as well as acting as contaminants which affect the electrical properties of the cellular insulation.
However the production of water also results in the presence of residual water in the cellular insulating coatings produced.
Water is also evolved directly by the thermal decomposition of other blowing agents such as p,p'-oxybis(benzene sulfonylhydrazide).
As discussed above in most fields of technology the amount of moisture produced in thermal decomposition of the commercially available particulate blowing agent would be considered insignificant even if the user was aware of it and in any event would be too small to produce any deleterious effect. This is not the case, however, in the sophisticated field of communication cable technology.
In any communication cable used at the higher frequencies (10 megahertz) which are frequencies for the video or television range, loss of signal becomes the important characteristics and even with the best manufactured communication cables there is a signal loss with distance (attenuation). This signal loss increases with frequency resulting in the need to introduce repeators (amplifiers). The attenuation is in part dependent on the dissipation factor and the SIC (Specific Inductor Capacitance) of the insulator in the transmission line. For example, a properly expanded cellular insulator which has not been put through a drying cycle, and which thus contains residual moisture, will have a dissipation factor of as high as 0.0006; after vacuum drying to remove the moisture, however, the dissipation factor will drop to 0.00015 and the attenuation of the line will drop accordingly.
The normal manufacture of CATV cables for the North American market invariably involves a prolonged drying cycle to remove residual moisture in the cellular insulation; this drying cycle is performed in an oven of low relative humidity (about 5%) at 150.degree. C. or in a vacuum bell at 150.degree. C., prior to application of the outer conductor and sheath of the cable. This moisture must be removed before applying the outer conductor, since otherwise the outer conductor will hermetically seal the moisture in after which it would be impossible to remove more moisture from the cable.
In conventional practice reels of the cables which may contain 15 to 20 layers on a reel are dried in ovens or vacuum bells as described for periods of the order of 48 hours.
Clearly, therefore, if the presence of residual moisture can be avoided or significantly reduced, the prolonged drying cycle can be eliminated, thereby resulting in considerable economy both in time and money.
Similarly the presence of inert particles of clay or metals as nucleation sites in the thermoplastic material will affect the electrical properties, for example, the capacitance of a wire or cable coated with the material.
In one aspect the present invention seeks to overcome these problems by providing a method and apparatus for producing an extruded cellular thermoplastic insulating coating on a communication cable conductor wire wherein a small amount of a heat decomposable material is used to provide nucleation sites for a gaseous material and the use of non-heat decomposable particulate materials such as clay and metals as nucleation sites can be avoided; in this case the residual non-gaseous thermal decomposition products formed from the decomposition of the small amount of heat decomposable material are at a minimum, and so any adverse effect is also minimized and the drying cycle can be reduced or minimized.
It is also conventional practice to modify thermoplastic polymer material by the incorporation of a crosslinking agent in the thermoplastic material, particularly in the power cable industry. The cross-linking agent is incorporated in the thermoplastic polymer material prior to the introduction of the material into the screw extruder. Generally, the supplier of the thermoplastic material incorporates the crosslinking agent in the material so that it will set when heated to an elevated temperature. This has inherent disadvantages in that the customer has no control over the content of cross-linking agent in the thermoplastic material supplied and individual batches from the supplier vary with consequent variation in the electrical properties of the coated wire or cable. Also, cross-linking agents are often extremely unstable and some cross-linking inevitably occurs during storage of the thermoplastic material containing the cross-linking agent and this makes the production of a homogeneous mixture more difficult during extrusion in the screw extruder.
In the conventional procedure the supplier of the polymer incorporates the cross-linking agent into the polymer while it is in a molten state and thoroughly mixes the molten polymer with the cross-linking agent under carefully controlled conditions in an attempt to minimize any cross-linking. After the mixing is complete the molten polymer containing the cross-linking agent is cooled and stored ready for shipment to the cable manufacturer.
This somewhat unsatisfactory procedure arises from the difficulty in handling molten polymers with crosslinking agents without prematurely initiating the crosslinking. The normal shearing action which occurs in a screw extruder is not adequate to produce thorough mixing of the cross-linking agent and attempts to improve the mixing by increasing the temperature of the molten polymer to lower its viscosity result in premature cross-linking which in turn results in blockages in the screw extruder, necessitating dismantling of the extruder for cleaning to remove cross-linked material.
The mixing means employed on the screw in the method of the present invention, however, overcome this difficulty and produce thorough mixing without premature cross-linking in the screw extruder.
In one aspect of the present invention there is provided an improved method and apparatus for producing a cross-linked polymeric extruded insulating coating in which the user is able to control the content of cross-linking agent directly.
It is an object of this invention to provide an improved method for producing cellularly insulated wires, particularly conductor wires for communication cables.
It is a further object of this invention to provide an improved method for producing a cross-linked insulating coating on a wire, particularly a conductor wire for a power cable.
It is a further object of this invention to provide a novel screw extrusion apparatus having a novel valve means and mixing means.